FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1950
1950 - 2072.PDF
512 FLIGHT, 7 December 1950 His first section was " Flight " photograph. '%M ANY misconceptions concerning flight researching**- the relatively new problems of njiiij, m liigtrflfllach Numbers should be removed by study of S/L. John Derry's paper, read before the Graduate Section of the Royal Aeronautical Society on November 30th, on high- speed flying from the test-pilot's point of view. The lecture, which attracted one of the largest audiences seen in the R.Ae.S. library, is sum- marized here under the five main headings chosen by the author. S/L. Derry prefaced his paper by reminders that he spoke as a pilot and not as a technician and that, for security reasons, most of his examples had been chosen from personal experience in air- craft now considered obsolete, entitled—: Obstacles to negotiate at high and low altitude.—It was possible, he said, to isolate the effects of compressibility from the problems associated with high air-loads. To some extent the desire of both pilot and aerodynamacist to initiate all research at high altitude sprang from the same source; for a given true speed, indicated speed—dynamic pressure— was halved at 40,000ft and the speed of sound was 100 m.p.h. slower above 35,000ft. Another factor, often not appreciated, which affected the pilot was that it was, and would be for some time, impossible to break a normally strong research aircraft or fighter above 30,000ft. Also, control-forces were low at altitude, facilitating quick correc- tion of load-increases caused by compressibility; and the pilot was able to abandon his aircraft without encountering the shattering effects of high air pressures. Experience had shown that compressibility produced the following general symptoms: variations of hinge moment; flow breakaway causing buffet; changes in stability; tab and control ineffectiveness; and large unpredictable trim- changes. The last two effects were the most serious, and had given rise to confusing popular ideas about the "barrier." The most troublesome trim-change was nose- down—for manual or powered controls. In both cases the final result could be a very steep dive, either because of the trimmer's inability to relieve stick-force sufficiently or of complete ineffectiveness of control. Thus, a serious nose- down trim-change could result in genuine loss of control, although its onset was rarely too quick for some form of corrective action. The author gave two instances of this vice. The first provided a reminder that few jets had exceeded the limiting Mach Number of the Spitfire (M=0.92, attained by S/L. Martindale, then of the R.A.E.). At M=0.83 in a Spitfire, a strong nose-down trim-change occurred, building up rapidly at M = 0.85 until full nose-up trim was insufficient. At M = 0.86 the aircraft could not be pulled out of the dive with a 150-lb stick-force. The uncontrolled dive lasted for a short time only because of the drag-increase caused by clos- ing the throttle in fine pitch. The second example con- cerned the D.H.108. In this case, an uncontrolled dive resulted from a serious loss in elevon effectiveness, which S/L. JOHN DERRY, D.F.C., joined the de Havilland AircraftCompany as an experimental test-pilot in 1947. In a D.H.108 experimental machine, on September 6th, 1948, he became thefirst pilot to fly a British aircraft faster than sound. Derry, who is 29, began his R.A.F. service in 1940 as a wireless operator/air-gunner. Later he trained as a pilot and flew rocket-firing Typhoons from 1944-45. After the war he served at the CentralFighter Establishment and, on his release, began his career as a civilian test-pilot with Vickers-Supermarine. demanded full "up" movement to trim at M=0.98. A relatively small nose-down trim-change at M=0.99 was enough to carry the aircraft into a dive. Additional trim was provided by trim-flaps, but these quickly became in- effective and only very prompt action prevented develop- ment of a vertical dive. There was no means of recovery until the terminal Mach Number fell below that at which control was lost. This condition was realized .at 30,000ft, at which the T.M. was 1.0; a slow recovery was completed at 25,000ft at M=0.95. These examples, illustrating the worst ' effects of Mach Number, showed that the extreme case was not necessarily disastrous. S/L. Derry did not know of any crash which could be directly attributed to loss of control at high altitude as a result of compressibility effect. How- ever, on more advanced aircraft, extra weight and lower drag might produce more serious results. Nose-up trim-changes, more irritating than dangerous, HIGH-SPEED FLYING —from a Pilot's Viewpoint : John Derry's R.Ae.S. Lecture produced, at the worst (at high altitudes), a " g-stall." This was not likely to prove catastrophic and, in practice, was rarely experienced unintentionally at high Mach Numbers. Swept-wing aircraft, exhibiting the now well-known tip- stall, entered this condition more suddenly; and for aircraft with spinning tendencies, the " g-stall" brought the risk of spinning. Fortunately, the " g-stall" was usually straighter than was the normal stall. Reduction of g gave immediate recovery. Lateral trim-change had become an innocuous but some- In America, the Bell X-l and Douglas Skyrocket (above) research aircraft have proved their worth as practical instruments of super- sonic flight-research. Both types employ rocket motors.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
